In traditional cellular type mobile communication systems, a mobile station is "handed off" from a servicing base station to a target base station when it appears that the signal quality with the target base station will be superior to the signal quality of the servicing base station. Typically "handoff" occurs when the mobile station is moving from the coverage area of the servicing base station into the coverage area of the target base station.
One type of handoff, commonly referred to as a "hard handoff", requires the mobile station to change the frequency and/or time slot of its communication signal when it is handed off from the servicing base station to the target base station. Commonly, a hard handoff is an asynchronous function in which the mobile station and the base station asynchronously perform a break-before-make switching. For example, an initial voice path is established between a mobile station and the servicing base station when the mobile station is in the coverage area of the servicing base station. When it is determined that the mobile station should be handed off from the servicing base station to the target base station, a handoff order is transmitted to the mobile station with instructions to switch to the frequency and/or time slot allocated for the target base station, and in response, the mobile station retunes to the allocated frequency and/or time slot and synchronizes with the target base station. Next, via signalling between the base station and the communication system, the target base station verifies the presence of the mobile station, and the communication system establishes a voice path to the mobile station through the target base station. The period of time between the handoff order and the reestablishment of the voice path through the target base station is commonly referred to as the handoff period and often is in the range of 1-3 seconds. The handoff period results in a loss of voice communications and degradation in voice quality that may exceed what is acceptable, particularly for mobile communication systems that are intended to replicate the quality of traditional fixed communication systems.
Another type of hard handoff occurs when the communication signal with the mobile station at a particular frequency and time slot experiences some type of disturbance or degradation not necessarily associated with moving from one coverage area into another coverage area. In this type of hard handoff, the mobile station retunes from its current frequency and/or time slot to a new frequency and/or time slot, without switching from the servicing base station.
More detailed descriptions of various mobile communication systems and the handoff procedures used therein are contained in "Wireless and Personal Communication Systems", Vijay K. Garg and Joseph E. Wilkes, published in 1996 by Prentice Hall PTR and "Cellular Radio Systems", D. M. Balston and R. C. V. Macario published in 1993 by Artech House Publishers, the disclosures of which are incorporated herein by reference.